Footwear manufacturing operations involve a multi-step procedure in which an upper, comprising an assembly of various components used to cover and support the top of the foot, is coupled to a sole. Outdoor multi-purpose boots, in particular, are typically constructed by assembling an outsole, a molded shell, an insole, and an upper. The molded shell is often formed by injecting a polyurethane or other suitable polymer into an appropriately shaped mold, where the polymer cures into a solid shell. Use of polymers such as polyurethane permits the manufacture of a shell that provides waterproofing, resiliency, and resistance to damage.
Prior art shells have been molded using a pair of mold faces which together form a cavity that defines the outer shape of the shell. During molding of a shell, the mold faces are joined and molding material is introduced into the cavity. Typically, some of the molding material escapes along the interface of the mold faces. This creates a feature that is commonly known as a flash line or parting line—a raised line on the surface of a molding. The flash line is generally undesirable, as it may detract from the aesthetic and/or functional qualities of the shell. Additional finishing operations may be required to remove the excess material so that it does not remain on the final product in a prominent location. In order to “hide” the flash line, some shell designs incorporate ribs or other protrusions and molded geometries coinciding with the location of the flash line. For example, the rib may be molded along the longitudinal axis of the shell, at the location where the mold faces were joined during injection molding.
As shown in FIGS. 1 and 2, a prior art polyurethane shell 8 may be produced using cooperating halves of a split metal die assembly 10. A left die half 12 mates with a right die half 14 to create a cavity 16 in a desired outer shell shape. Die halves 12, 14 are configured and dimensioned such that each die half defines either the lateral portion 12′ of shell 8 or the medial portion 14′ of shell 8. A last or inner mold portion 18 is disposed between die halves 12, 14 in cavity 16, thereby creating a desired inner shell shape. Cavity 16 of die assembly 10 is in communication with runners 20 ending in gates 22, through which polyurethane flows for injection into cavity 16. After polyurethane is injected into and fills cavity 16 in the space defined between die halves 12, 14 and inner mold portion 18, sprues may be created integrally with the shell as a remnant of the molding process.
As shown in FIG. 2, a flash line 24 is disposed along the longitudinal axis 26 of shell 8. Flash line 24, for example, is generally in the form of a protrusion extending above the adjacent surface 28 of lateral portion 12′ and adjacent surface 30 of medial portion 14′. Because of the mating of die halves 12, 14, flash line 24 is disposed in both the toe region 32 and the heel region 34 of shell 8. Especially proximate toe region 32, flash line 24 provides a stiffened region of shell 8 proximate longitudinal axis 26, which may adversely impact performance of shell 8 by limiting flexure when disposed around a foot of a user.
In view of the prior art shell and manufacturing technique therefor, there exists a need for an improved shell and manufacturing method. In particular, there exists a need for shells molded such that the flash line is positioned along a perimeter thereof, instead of along the longitudinal axis, so that flexure proximate the axis is not hindered by the flash line. There also exists a need for a method of manufacturing shells which obviates the need for performing extensive finishing operations on the outer surface of the shell that is exposed in the finished boot.